This invention relates to the driving of electroluminescent (EL) lamps to be used as backlights for timepiece displays. More specifically, the invention relates to a driving circuit that uses a converter to progressively charge a capacitive EL lamp with a predetermined number of successive current pulses.
EL panels (lamps) having different characteristics and electronic circuits used to drive the lamps are shown extensively in the prior art. Generally, EL lamps are characterized as lossy capacitors that require ac excitation signals having amplitudes such as 40 to 80 volts, peak-to-peak, which are larger than can be provided by a timepiece battery. See U.K. Patent Specification No. 1,382,256 (Sliker), Technical Documentary Report No. RADC-TDR-64-159 prepared by RCA for the Air Force Systems Command and U.S. Pat. No. 4,238,793 issued to Hochstrate. Copending U.S. patent application Ser. Nos. 362,708, 514,078, 417,779, and 490,621, all having the same assignee herewith, disclose EL lamps and drive circuits.
As a specific example, U.S. Pat. No. 4,011,557 issued to Chetelat et al. discloses an EL watch display which is run from a small battery. The EL watch display is fed by short impulses of high-intensity current by means of discharges from two capacitors which are alternately charged and discharged by transistor switches opened and closed by externally supplied impulses. In this case, EL diodes (a type of LED) are used to form the display symbols. However, current is not continually fed to each LED which is to be lighted. Each LED is impulse-operated at a frequency exceeding 30 Hz. The impulses are provided by two capacitors which are alternately charged and discharged. In effect, the energy in the field of a capacitor is discharged across a capacitive EL lamp, i.e. an EL display diode. The capacitors serve as a high-voltage supply for charging each capacitive EL display diode depending upon the state of the transistor switches. However, every time an uncharged EL display diode is charged from a capacitor, energy is dissipated in the resistances of the switches and the capacitive EL display diode itself. Therefore, recharge efficiency is low due to this dissipation of energy.
It is an object of this invention to provide a high efficiency drive circuit that delivers directly to a capacitive EL lamp substantially all of the energy produced by the drive circuit.
Another object of the invention is to provide a drive circuit that converts power efficiently to EL lamps having different lamp characteristics.
Another object of the invention is to provide a drive circuit for driving an EL lamp used as a backlight in timepiece displays.
It is another object of the invention to provide a drive circuit that drives an EL lamp from a timepiece battery using a converter that steps up the low battery voltage.
An object of the invention is to provide a drive circuit that progressively charges an EL lamp, which acts as storage capacitor, with successive current pulses provided by the converter.
Yet another object of the invention is to alternate EL lamp polarity at the end of a predetermined number of successive lamp charging current pulses.
It is still another object of this invention to provide a drive circuit that includes a converter that has an inductor for stepping up the timepiece battery voltage to successively higher voltages that progressively charge the capacitive EL lamp at each of two alternate polarities.
Still another object of this invention is to provide high EL lamp light output from a single-cell timepiece battery at low current drain.
A further object of this invention is to use an inductor in the drive circuit for energy exchange with the capacitive EL lamp.